This invention relates to an ultrasonic transducer. More particularly, the invention relates to an ultrasonic transducer comprising a support body, a piezoelectric vibrator arranged on the support body, a first matching layer attached at least indirectly to the piezoelectric vibrator, and a second matching layer attached at least indirectly to a surface of the first matching layer facing away from the piezoelectric vibrator. The piezoelectric vibrator consists of a material having a relatively high dielectric constant and a high acoustic impedance.
Wideband ultrasonic transducers are well known in the fields of ultrasonic medical diagnostics and in the field of non-destructive materials testing. The medical applications, where coupling between body tissue and a sound transducer must be accomplished with a minimum of losses, in particular require improvement of the electromechanical and acoustic properties of ultrasonic transducer systems.
To couple a piezoelectric ultrasonic transducer with an acoustic impedance Z.sub.0 of, for example, 29.times.10.sup.6 kg/m.sup.2 .multidot.s wideband to a load such as water which has an acoustic impedance Z.sub.L of about 1.5.times.10.sup.6 kg/m.sup.2 .multidot.s, one or more matching layers can be arranged between the piezoelectric vibrator and the load. In the literature (IEEE Transactions on Sonics and Ultrasonics, Vol. Su-26, No. 6, November 1979, pages 385 to 393), the use of so-called quarter wavelength (.lambda./4) matching layers is recommended for the design of wideband, low-loss ultrasonic transducers. From theory, equations are obtained for determining the acoustic impedances of the interposed matching layers. If only a single quarter wavelength matching layer is used, the optimum value for its acoustic impedance is given by the equation Z.sub.1 =.sqroot.Z.sub.0 .multidot.Z.sub.L. In an ultrasonic transducer with piezoelectric vibrator consisting of ceramics or lithium niobate (LiNbO.sub.3) and with a load of water, an acoustic impedance of about 6.6.times.10.sup.6 kg/m.sup.2 .multidot.s is obtained for the quarter wavelength matching layer. If two quarter wavelength matching layers are arranged between the ultrasonic transducer and the load, the optimum value for the acoustic impedance of the first quarter wavelength matching layer is approximated by the equations Z.sub.1 =.sqroot.Z.sub.0.sup.3 .multidot.Z.sub.L and the second quarter wavelength matching layer by the equation Z.sub.2 =.sqroot.Z.sub.0 .multidot.Z.sub.L.sup.3. For a piezoelectric vibrator of ceramics or lithium niobate with an acoustic impedance Z.sub.0 of 29.times.10.sup.6 kg/m.sup.2 .multidot.s, acting on a load with an acoustic impedance Z.sub.L of 1.5.times.10.sup.6 kg/m.sup.2 .multidot.s, an acoustic impedance Z.sub.1 of approximately 13.8 kg/m.sup.2 .multidot. s is obtained for the first quarter wavelength matching layer and an acoustic impedance Z.sub.2 of approximately 3.1.times.10.sup.6 kg/m.sup.2 .multidot.s for the second quarter wavelength matching layer. These theoretical values are valid only for a single frequency. It is therefore possible that wideband ultrasonic transducers having layer thicknesses and acoustic impedances which differ slightly from the theoretical values can exhibit good transmission properties. Thus, one can use for the first quarter wavelength matching layer, for example, quartz glass (Z=13.1.times.10.sup.6 kg/m.sup.2 .multidot.s) and for the second quarter-wavelength matching layer, for example, polymethacrylic acid methyl ester PMMA (Z=3.2.times.10.sup.6 kg/m.sup.2 .multidot.s).
In a known ultrasonic transducer wherein a ceramic transducer is matched by two quarter wavelength matching layers to a load medium such as body tissue or water, a backing of epoxy resin with an acoustic impedance of approximately 3.times.10.sup.6 kg/m.sup.2 .multidot.s is used. A first quarter wavelength matching layer consists of a glass with an acoustic impedance of approximately 10.times.10.sup.6 kg/m.sup.2 .multidot.s and a second quarter wavelength matching layer consists of polyacryl or of epoxy resin with an acoustic impedance of approximately 3.times.10.sup.6 kg/m.sup.2 .multidot.s. The glass plate, i. e., the first quarter wavelength matching layer, is fastened by a cement adhesive of very low viscosity to the ceramic transducer body. The thickness of the adhesive layer is approximately 2 .mu.m. The epoxy resin, i.e., the second matching layer, is cast directly on the first matching layer ("Experimental Investigation on the Design of Wideband Ultrasound Transducers", Biomedizinische Technik, Vol. 27, Nos. 7 to 8, 1982, pages 182 to 185). This double quarter wavelength matching layer results in an improvement of the bandwidth of the ceramic transmitting layer. The bandwidth of this ultrasonic transducer is approximately 60 to 70% of the center frequency.
In another known ultrasonic transducer which contains a transmitting layer of a material with a relatively high dielectric constant and high acoustic impedance and two quarter wavelength matching layers, the first matching layer, which faces the transmitting layer, has an acoustic impedance of approximately 14.times.10.sup.6 kg/m.sup.2 .multidot.s and consists exemplarily of porcelain, especially a vitreous material (Macor), and preferably of quartz glass (fused silica). The second quarter wavelength matching layer, facing the load, has an acoustic impedance of approximately 4.times.10.sup.6 kg/m.sup.2 .multidot.s and consists exemplarily of polyvinylchloride (PVC) and in particular, of polyvinylidene fluoride (PVDF).
The second matching layer also serves as the receiving layer. In addition, the first matching layer is provided as backing for the receiving layer. By this design an ultrasonic transducer with a low-reflection transmitting layer matched throughout a wide band to a load and a sensitive and wideband receiving layer is obtained. (See European Offenleounqsschrift No. 0 118 837.)
In these known ultrasonic transducers, the bandwidth was improved by means of quarter wavelength matching layers, chosen because they had desired acoustic impedance values obtained from theory. It is known from the literature that the number of materials usable as matching layers is limited and that other material properties such as mechanical machinability are relegated to a background role. Such other material properties, however, are important in the design of compact linear or matrix-shaped ultrasonic transducer systems.
An object of the present invention is to provide an improved ultrasonic transducer of the above-described type.
Another, more particular, object of the present invention is to provide such an ultrasonic transducer which has a piezoelectric vibrator matched acoustically throughout a wide band to body tissue or water.
Another particular object of the present invention is to provide such an ultrasonic transducer having a first matching layer which can be machined in a simple manner to give it shape.